Since yesterday Microsoft has released pubCenter as a beta to the public. pubCenter is an ad publishing tools that lets you place contextual advertisements on your sites. It was already being used, but only for a select group of testers. Now that Microsoft has released it to the public it is looking for more participants to place advertisements on their site. You can compare pubCenter to Google AdSense which is Google’s way of placing adverts on your personal site.
If you are interested in the program and you are a resident of the US (because for the time being the beta is restricted to US residents only) you can sign up for free on Microsoft’s avertising link.